In general, a VCO is an oscillator with a variable capacitor whose capacitance is changed through voltage adjustment, to thereby adjust the frequency. Such a VCO is an essential component of all electrical communication systems and may be used for up-converting or down-converting a frequency of a predetermined signal.
FIG. 1 is a circuit diagram of a conventional LC VCO.
Referring to FIG. 1, the conventional LC VCO 100 includes an LC resonant circuit 130 which oscillates at a frequency depending on a control voltage VC, and an amplification circuit 150 which amplifies the oscillation frequency output from the LC resonant circuit 130.
In the conventional LC VCO 100 configured in such a manner, an NMOS transistor NM and a PMOS transistor PM are connected in series to both ends of an inductor L so as to form one current path. Therefore, the VCO can oscillate while reducing current consumption by one half.
In the LC VCO 100, however, since the NMOS transistor NM and the PMOS transistor PM are not exactly symmetrical with each other, an output is unbalanced. To solve this problem, a resistor Rs is connected to an output stage to equalize output levels. However, the magnitude of a supply voltage should be increased as much as a voltage drop caused by the resistor Rs.
Meanwhile, when a quadrature VCO is implemented by connecting two such LC VCOs through a switching transistor, the switching transistor is additionally connected to an oscillation load such that current consumption increases.
Further, the switching transistor and a substrate are easily coupled to each other such that a parasitic component of the substrate is directly fed-back to the oscillation load. This degrades a phase noise characteristic.